Recently, a superconductive oxide of the bismuth-calcium-strontium-copper oxide system was proposed, and research and development efforts have been made for target formed of the superconductive oxide, because the sputtering technique is an attractive process for formation of a superconductive thin film in various practical applications.
A typical example of the process of fabricating the target starts with preparation of ingredient powders of the bismuth oxide (represented by the molecular formula of Bi.sub.2 O.sub.3), the calcium carbonate (represented by the molecular formula of CaCO.sub.3), the strontium carbonate (represented by the molecular formula of SrCO.sub.3) and a copper oxide (represented by the general formula of CuO). These ingredient powders are mixed in a predetermined proportion to produce a mixture, and the mixture is calcined at 700 degrees to 800 degrees centigrade for a predetermined time period. The product is then pulverized. These steps, i.e., the calcining and the pulverizing are repeated two or three times, and a powder of the superconductive oxide of the bismuth-calcium-strontium-copper oxide system is finally obtained. The powder of the superconductive oxide is molded by using a usual pressing technique and, thereafter, sintered to produce a target. The powder of the superconductive oxide may be alternatively shaped by using a hot pressing technique to form the target.
However, a problem is encountered in the target produced through the prior art process in that cracks are much more liable to take place in the target during the sputtering. This is because of the fact that the superconductive oxide of the bismuth-calcium-strontium-copper oxide system has low thermal conductivity. Since the target is cooled off at one surface thereof and subjected to the bombardment of plasma at the opposite surface during the sputtering, a large difference in temperature takes place between both surfaces due to the low thermal conductivity. The large difference in temperature is causative of a thermal stress and, accordingly, the cracks. Thus, the prior art target suffers from a relatively short life time, which increases the production cost of a thin superconductive film.
Another problem inherent in the prior art target is a low electric resistivity. Therefore an expensive RF sputtering system is necessary for the prior art target, and a DC diode sputtering system is hardly usable. The expensive RF sputtering system also increases the production cost of the thin superconductive film.
Still another problem inherent in the target fabricated through the prior art process is low mechanical strength, and, for this reason, the prior art target is liable to be broken by ill usage. This also decreases the life time of the target and, accordingly, increases the production cost of the thin superconductive film.
The prior art target has still another problem; a relatively low critical temperature or a zero-resistivity temperature.